Silicon-alloyed corrosion-resistant metal powders and related products and processes



Patented Oct. 27, 1953 UNITED STATES OFFICE SILICON-ALLOYED CORROSION -RESISTANT METAL POWDERS AND RELATED PROD- UCTS AND PROCESSES tion of Maryland No Drawing. Application March 31, 1950, Serial No. 153,288

Claims. 1

This invention relates to readily compactible and moldable powders which resist corrosion, shaped bodies of such powders, and methods for producing the same, some aspects of the invention being of a broader scope.

Among the objects of the invention are powders of metal particles which are in a readily deformable plastic condition and may be compacted into relatively strong metal bodies with relatively low pressure, at least the surface layer of the metal powder particles having alloyed therewith silicon from a gaseous or liquid silicon compound at an elevated temperature.

A particular object of the invention is such readily compactible moldable powder containing essentiall powder particles of at least one metal of the group consisting of iron, nickel and cobalt, and of mixtures and alloys of said metals, which particles are in a readily deformable, relatively soft plastic condition, at least the surface layer of said particles having alloyed therewith silicon which has been deposited thereon at an elevated temperature from a silicon compound.

Another object of the invention is such siliconsurface-alloyed soft moldable metal powder combined with similar surface-alloyed powder particles of at least one metal of the group consisting of molybdenum, tungsten, chromium, vanadium, manganese, or mixtures or alloys thereof.

Further objects of the invention include compacted bodies made of such surface-alloyed metal powder particles, methods for the economical production of such surface-alloyed soft moldable metal powder particles, and economical production of compacted bodies from such powder particles of the invention.

The foregoing and other objects of the invention will, be best understood from the following description of specific exemplifications thereof.

For years past various corrosion-resistant products such as gears, valve parts, bearings, filters, and the like, have been made of stainless steel powders by powder metallurgy techniques. The stainless steel powders generally used for such bodies have been obtained from previously prepared stainless steel ingots, for instance, by subjecting a stainless steel ingot to grain boundary corrosion followed by disintegration, or by melting a stainless steel ingot and atomizing it in the molten state. The so-produced particles of such stainless steel powders are of high density and hardness, and require high-molding pressure of at least 40 to 45 t. s. 1. (tons per square inch), which is much too high for economical produc- 2 tion of molded products, and results in rapid wear and frequent break-down of the molding dies. As a result, the production of molded products from stainless steel powders has found only limited use.

What has been said above about stainless steel powder, which is an alloy of iron, nickel and chromium, applies also to alloys of iron, cobalt and nickel with aluminum.

According to the invention the foregoing difficulties heretofore encountered in the production of corrosion-resistant ferrous metal bodies by powder metallurgy techniques are overcome by the provision of a readily compactible and moldable metal powder of relatively soft ferrous powder particles, at least the surface layer of the powder particles having combined therewith silicon which has been deposited thereon from a silicon compound at an elevated temperature so as to render the powder particles corrosion-resistant, the surface-alloyed metal powder particles being sufiiciently soft and plastically deformable to make it possible to compact them with relatively low pressure, for instance, only 25 to 30 t. s. i. into bodies of even greater strength than molded bodies made by compacting prior art powders of corresponding composition with a molding pressure of at least 40 to 45 t. s. 1.

Various commercially available soft, plastically readily deformable iron powders are suitable for producing corrosion-resistant soft moldable metal powders of the invention. Among such available and suitable soft iron powders is sponge iron powder having a carbon content of .l to .2% (unless otherwise specifically stated, all proportions are given herein by weight), electrolytically produced iron powder, carbonyl iron powder, eddy mill iron powder, and atomized molten iron powder of silimar carbon content. The following impurities present in such available powders, to wit, 3% Mo, 2% Si, and other minor impurities usually present therein, do not impair their usefulness for practicing the invention.

In addition, many relatively hard and difiicult to compact fine iron powders containing up to about 2% carbon will, in general, when subjected to the silicon-alloying process of the invention herein described, become softened and acquire characteristics which render them readily moldable.

In accordance with a phase of the inventionbased on the original concept of producing si1icon-surface-alloyed ferrous iron powderrelatively soft powder particles of different metals, to wit, one or more metals of the group including iron, nickel and cobalt, are combined in proper proportion, and a desired proportion of silicon from a gaseous compound is deposited on and diffusedfintothe interior of such metal powder particles so as to yield soft powder particles combining silicon with the other metal or metals in proportions required for producingalloyed bodies of desired characteristics andfco'nt'aining silicon and such metals in the desired proportions.

Any of the known methods for depositing sili-- sicli-pnz si ncl 1) or' according to the equation *siCLi-l-metal si i-m'et'al chloride (2) or simultaneously according to both these-equations.

In any case, the silicon:atomstof the gaseous silicon compound are deposited on the surface of the metal powder particles, and diffuse into the interiorand alloy with metal of the particles at the temperature of the treatment.

I In accordance with one phase of the softiron particles are first sinteredinto sinter cake pieces withina reducing temperature, such as pure dry hydrogen, or purified dry cracked ammonia, at anelevated temperature so as to produce a porous sinter cake or low density, not higher than about 4 to 5 g./cc., and of low strengthnot higher than modulus of rupture of about 65.00 to 7000 p. s. i. The powder particles of such porouslow density sinter cake pieces pro duced by the initial sintering treatment, may be readily surface-alloyed with silicon by known processesfordepositing silicon from a gaseous or liquid silicon compound at an elevated temperatureat which the deposited silicon diffuses into the interior of the powder particles. The porous low density sinter cake pieces subjected to such silicon-surface-alloying treatment do not undergo any material increase in density and strength when subjected to this surface-alloying treatment, and they may be readily comminuted with minimized work hardness into minute soft powder particles of therequired size range and high degree of plastic deformability which makes it possible to compact the resulting silicon-surfacealloyed ferrous powder particles with a pressure of only 50,000 p. s. i. into strong green metal powder compacts having a modulus of rupture of 400 p. s. i. or more, without adding any binder to the powder particles.

In accordance with another phase of the invention, powder particles of the oxides of ferrous metals',such as iron, nickel and cobalt, or mixtures of such oxides, or mixtures of such oxides and metals-with or without additional mixtures of powders of oxides'of metals of the group consistingof molybdenum, tungsten, chromium, vanadium, and manganese, or powders 'of" these metals, or mixtures of powders of these metals and their oxides-are initially sintered into pcthe invention,

rous sinter cake pieces at an elevated temperature within a reducing atmosphere so as to produce sinter cakepieces of the reduced metal pow- ,(der particles having 'a low density not higher than about 4 to 5 g./cc., and of low strength not higher than a modulus of rupture of about "7000 p. s. i. The metal powder sinter cake pieces obtained by s'nch initial reducing treatment are "then surface''alloyed with silicon from a silicon 'cor'npound'at an elevated temperature at which the deposited silicon diffuses into the interior of the metal powder particles. The low-density sinter cake] pieces subjected to such silicon-surface alloying treatment do not undergo any material 'increasein'density or strength, and may be readilycomminuted into minute powder particles of the required size range which have only a minimized work hardness, and exhibit a high 'degree "of softness and deformable plasticity which makes it possible to compact the resulting silicon surface alloyed minute metal powder particles with'a pressure of only 50000 pf s.i.into strong green powder compacts having a modulus of rupture of 400 p; s. i. or more, without adding any binder to the powder particles. The s'u'rface-siliocn-alloyed metal powders produced in the manner described above from oxides'of the different metals will yield a surface-alloyed powder body, the individual particles of whichare alloys of the different metals and substances corresponding'to thedifferent metals'or oxides used for making the initialsinter cake pieces and the proportion of silicon deposited on the sinter cake particles.

"A convenient way for producing such sinter cakepieces for the foregoing treatment is to mix thepowders of the metal oxides with a lubricant -and binder andpellet the mixture into pellets, the pelletslcon'taining' enough of the lubricant and. binder which decomposes and is driven off 'at 'the "smtering temperatur so that the resulting sinter cake pellets or pieces have the required high porosity and the required low densityand low strength.

Without in any way limiting the sc'opeof'the invention, there will be hereinafter 'describedexamples of satisfactory procedures for producing soft silicon-surface-alloyed metal powders of the invention from commercially availab1e'raw'ma terial.

There will now be described, by way 'of example, a satisfactory procedure for producing silicon-'surface-alloyed corrosion-resistantsoft iron powder "of the invention from commercially avai1 ablesponge iron powder having a carboncontent of about .15%.

A typical commercial powder of this type'used 1n the'process'consisted of a mixture of minute powder particles .having the following particle- :to inch is deposited into sinter'boats'of suitable heat-resistant metal such as iron-chromium sheet metal, without inany way compactingthe deposited iron powder. Theinterior"surface-of each'isinterb'oat is coated with attics-suppressing or release medium such as a water suspension of magnesium hydroxide to permit ready separation of the sintered powder body from the boat after the sintering operation. The powder within the boat is then treated within a furnace in a reducing atmosphere, such as purified dry hydrogen or purified cracked ammonia, at a temperature of about 800 to 1050" C. for one-half to three hours, and then permitted to cool within the same atmosphere to room temperature. In general, such treatment at a temperature between about 900 to 1000 C'., such as 950 C., for one hour is sufficient for sintering commercial sponge iron powder into a sinter cake having a density of 1.7 to 4 g./cc., which is readily frangible into lumps or sinter cake pieces suitable for undergoing the desired silicon-surface-alloying treatment.

A good way for carrying on the foregoing sintering treatment is to push the powder containing boats through a tunnel furnace within which is maintained the desired reducing atmosphere, the heat treatment being followed by cooling within the same atmosphere as a part of a continuous process, in the course of which the boats are moved from one end of the furnace to the other.

After completion of the initial sintering treatment, the iron powder sinter cakes are removed from the boats and are broken into sinter cake fragments or pieces of a size suitable for the subsequent siliconizing treatment. Sinter cake pieces having cross-sectional dimensions in the range between about inch to about 1 ,4; or inch have been found very suitable for the subsequent surface-alloying treatment.

An example of a satisfactory procedure for surface-alloying such iron powder sinter cake pieces with silicon will now be described: The porous iron powder sinter cake pieces are packed within a pack mass which is eifective in depositing silicon on the sinter cake powder particles. The pack may consist, for instance, of 50% by volume ceramic lumps, such as porcelain pieces, the balance a silicon alloy such as ferrosilicon containing about 40 to 70% (by weight) silicon.

The sinter cake pieces packed with porcelain and ferrosilicon are placed within treatment baskets of suitable heat-resistant metal, such as a chromium-nickel-iron alloy, and the packed treatment baskets which are placed in a retort. The contents of the retort are heated to an elevated temperature in the range between about 900 and 1100 C., and an atmosphere contain ng pure dry hydrogen and dry hydrogen chlor de gas, is passed through the retort for producing reactions causing silicon atoms to be deposited on the metal powder particles of the sinter cake lumps, and to diffuse into the interior of the metal particles and alloy therewith. In such surface-alloying treatment, the hydrogen chloride gas passing through the retort interacts with the ferrosilicon to form silicontetrachloride gas. The silicontetrachloride gas is decomposed at the surface of the powder particles, and the silicon atoms deposit on the surface and difiuse into the interior of the metal powder particles and alloy the metal of the particles. i zcording to the silicon content in the treated powder, the time of treatment may be varied between about 10 and 20 hours. The temperature of the treatment may be varied between about 900 and 1100 C., and must be well below the melting point of the ferrosilicon of the pack.

By way of example, the following surface-alloye ing procedure gave satisfactory results: lhe broken up sinter cake lumps were packed in circular baskets having a diameter of 22 and at height of 10". Four such baskets were stacked upon each other in a closely fitting retort of 60" height. A mixture of 3 parts of gaseous H01 and 20 parts of Hz was passed at 1050 C. through the retort, first for 2 hours at a rate of 40 cu. ft. per hour, and then for 10 more hours at a rate of 20 cu. ft. per hour. The foregoing treatment yielded iron powder sinter cakes with a silicon content of about 12%. After being subjected to such silicon-surface-alloying treatment, the resulting sinter cakes have about the same density and the same strength or modulus of rupture which they exhibited prior to the silicon-surface-alloying treatment. Because of the low density and strength of the silicon-surface-alloyed sinter cake: pieces, they may be readily comminuted with. minimized work hardness into soft metal powder particles of a size range and high degree of plastic deformability required for compacting the: powder particles into strong pellets with relatively low pressure of only 50,000 p. s. i. into green; compacts having a modulus of rupture of at least.

200 and higher.

By way of example, surface-alloyed iron powdersinter cakes prepared in the manner described,

above, were disintegrated'by a disc crusher into ance 200, +325 mesh particles.

inch) The desired surface-alloying action may be: obtained without packing materials by the fol- The sinter-cake pieces are placed in a retort and a stream of hydrogen. and a gaseous silicontetrachloride is passed" through the retort while its contents are main- Satisfactory" results were obtained by passing gas mixtures; containing from 4 to 30% (by volume) of SiCh= through the retort at temperatures between. about 900 and 1160 C., and at rates in the; order of 30-60 cc. per second for periods ranging; The desired gas mixture was; produced by bubbling hydrogen through liquid; silicontetrachloride at room temperature, at; which the vapor pressure of SiCli is comparatively high so that sufficient amounts thereof are.

lowing procedure.

tained at an elevated temperature.

from 2 to 15 hours.

carried with the hydrogen into the retort.

The silicon-surface-alloyed iron powder particles of the invention, produced by the treat-- ment of the invention, have a spongy character,. tentacle-like shape. Green compacted bodies produced from such powders exhibit excellent corrosion-resistance under prolonged salt-spray tests, and they also resist attack by boiling dilute HNOa and H2SO4.

Similar results are obtained with other types of soft, plastically deformable iron powders subjected to the silicon-surface-alloying treatment of the invention. In fact, as explained above, many relatively hard and difiicult to compact iron powders will, in general, when subjected to the silicon-surface-alloying treatment of the invention of the type herein described, become softened and acquire the high degree of plastic deformability which renders them readily moldable into strong green compacts of high strength of at least 200 p. s. i.

' 'rhef foregoing-rupture test data. were obtained by test equipment and test methods described in the artiele'of J'P; Scanlan and R. P. Seelig in 'PowdenMetallurgy Bulletin, 44, p., 128 (1949) I using, test bars 1 /2 long, wide; 'and- 1/ thick.

least two to four times greater thansimilar prior art stainless steel powders. I I i A distinguishing characteristic of the 'corro si en-resistant silieon-suriace+alloyed soft 'fer rous "powders of' the invention is the factthat when'compacted into a green test bar body of. I the dimensions given above under pressure, of

' '25 s'.'i'.with no lubricant or binder, such cqmi pacted green bodyexhibits a modulus of rupture several times 'gre 'aterthan a similar body pro-,-

' 'duced by Compacting prior art'silicon steel pow: ders, under-the same pressure. In particular,;

such green test bar compactsmade from powders onus invention have a modulus ofrupture of at least 3o!) p. s. i.

In producing surface ialloyecl soft, plastically, readilydeiormable ferrous powders of the i vention, it is important that at all sta es of the I processirigj operations thepowder particles should notibe subjectedto any material work hardening I I forces, Thus, for instance; it is .e'ssential that action should not be compacted under any substantial pressure If a substantial compacting pressure, even as low as 5 t. s. i. is initially applied to suchsoft ferrous metal powder (Fe, Ni, Co) in preparation for the initial sintering process, which precedes the silicon-alloying treatment such silicon-alloyed sinter fragments will acquires, relatively great density and strength, andlarge forces will be required for crushing them, and the resulting powder particles will beseverely work hardened. Unless the sinter cake fragrnents which are to be subjected to the silicon-alloying treatment are of low density and strength -obtainabl e if the powder particles subjected'to the, preliminary sintering action have not been initially compacted under pressurethe crushing energy required for pulverizing the silicon-alloyed sinter cake fragments would be so large that the resulting silicon-alloyed powde 1 particles would be distorted in shape and work hardened, making it necessary to apply undesirably large pressure for compacting them int bodies oi therequired final shape.

other words when producing silicon-surface-alloyed spit ferrous powders of the invention, it is essential that the sintered powder cake fragments or lumpsproduced in preparation for the silicon-alloying treatment-should have aflqwfd ns" and corr sp 0 strength, s as-to its n w rk, ar en g impartedto the powd r par icles, when pul r zine the sinteredbowder: cakeimsm nt or um s fo wing the silicon-alloying treatment, thus resulting in powder particles exhibiting only minimized ininitiall'y sintering the soft iron powder particles I i into porous sinter cake fragments suitable for packing into the silicon-alloyingpack, the powder h h is ito be subjected to the initial sintering,

j I I By controllin I workhardening. Furthermore, it is also essential that the sintered powder cake'fragments or lumps-produced in preparation tor the; silicone, I I alloying treatment-should have high porosity so that they arepermeable to the gases by means I of which the silicon-alloying treatmentis car-x ried on.

' As explained the density and strength of the I sinter cake lumps-produced I in. preparation for, thesilicon-alloying treatment-it is possible to oontrol the preparation of soft, plastically de- I formable silicon-'surface-alloyed powders of the I invention. I y

above, the proper-range. of the density of the sinter cake lumps-eproduced in is aboutl.5 to 4.0 g-./ce. The modulus of rupture preparation for the silicon-alloying treatment, I

of such sinter cake lumps is correlated to their density, being-aboutfil) p. s. i. for sinter cake I lumps havinga density of-1.7 @s/CC-Ellld increas' I I I p 'ing' to about I 6500 p. s. 1.; for; sinter cake lumps I having a; density of 4.0'g./cc.

I ;Furthermore,;aslong as the-sinteroake lumps, I p l c produced'in preparationfor the silicon-alloying,

treatment, are of low density, and have alow I, 'modulusof rupture'in the rangesIset forth above, 1 l 'they'willalso have about the same density and j the same modulus of rupture after being sub- 'jectedto the silicon-alloying treatmentwherel p by siliconis surfaeealloyed with the individual I powder particles of suchsinter cakes.

Accordinglmto' obtain silicon-alloyedrpOWders I cakes, or sinter cake-lumpsproduced in prep- 7 of the invention, it is sufficient, to, control the i -density and/or modulus of ,rupture'of the sinter 'aration for the I silicon-alloying treatment.- As

long as the density of such sinter cake lumps,

is not more than about 4.0 g cc., and their l'l'lOd-r p. s. 1. they will, after the'silicon-alloying treat strength as to permit their pulverization without material work hardening of the powder particles. As a result, the silicon-alloyed powder obtained from such sinter cakes will have the desired high degree of softness and plastic deformability as to make it possible to compact such powders into green compacts having a modulus of ruptureof at least 300 p. s. i. and higher.

By sintering such green compacted bodies of silicon-alloyed soft iron powder of the invention, compacted with only arelatively small essure, there. are obtained bodies having the same, strength as those produced by sintering prior art metal powders of corresponding composition which have been compacted with much higher pressure. Good results are obtained by sintering green compacted bodies of such silicon-alloyed powder at a temperature in the range oi 1,200? to 135.0 C. within a, protective atm s her such s ry hy r g r f p r fled cracked ammonia. It is also desirable to maintain the PlfQtective atmosphere at a dew point of about -50 C. or below. Such low dew point atmosphere maybe obtained by a suitable getter such as pure vchromium powder or ferrosilicon, powder Cr) mixed with aluminum oxide.

Green compacted bodies made from soft silicon-surface-alloyed iron powder of'the invention maybe infiltrated with other metals such as copper and copper alloys for producing composite bodies havingthe desired combined characteristics. Such infiltratedbodies may be subjected to heat treatments; either at the time of ulusoifgrupture is not more'thanaboutfiooo I the infiltration, or subsequent thereto for causing the infiltrant and the metal particles and substances of the powder compact skeleton to difiuse into each other to any degree as desired.

According to a further phase of the invention, silicon-surface-alloyed soft plastically readily deformable powders of either iron, nickel or cobalt, or alloys or mixtures thereof, are produced in a very economical way from the oxides, or mixtures of the oxides of these metals, or mixtures of these metals and their oxides. Among the oxides suitable for this purpose is black mill-scale containing principally iron oxides (Fe3O4 and FeO) which is formed when rolling or forging iron and steel,

For producing silicon-surface-alloyed powders of the invention from oxides of iron, nickel or cobalt, or mixtures of the different oxides, the desired metal oxide or oxides may be subjected to a combined reducing and sintering treatment.

By way of example, there will now be described a satisfactory process for producing a silicon-alloyed powder containing soft moldable iron and. nickel alloyed powder particles partially silicon-alloyed by surface-difi'usion of silicon in accordance with the process of the invention.

Powder particles of iron oxide mill scale containing essentially F620: and FeO are mixed with nickel oxide powder and lamp black, and ball milled into a powder of 100 mesh. As an example, 200 parts of the mill scale are mixed with 22.6 parts nickel oxide and one part of lamp black. The powder mixture is then ball milled to powder of 100 mesh, and placed in treatment boats of heat-resistant metal coated on the interior with a stick-suppressing medium, and treated in a furnace under reducing atmosphere such as dry hydrogen or cracked ammonia at a temperature in the range between 800 and 1100 C. for one-half to three hours, and then permitted to cool in the same atmosphere. Satisfactory results are obtained by such treatment carried on at a temperature of about 950 C. for one hour. The powder mix may be pelleted into pellets before subjecting it to the foregoing treatment, or it may be placed into the treatment boat as a thin powder layer about A; to /4" deep, in which case the resulting sinter cake produced by the reducing heat treatment is broken up into lumps. The powder is formed into pellets by mixing it with a lubricant and binder so that the resultant mixed powder mass may be readily made up into small porous pellets, for instance, of cylindrical shape, having a diameter of about to inch, and the same hei ht, with a pellet density of 3 to 4 g./cc. Any suitable organic lubricant and binder which decomposes at elevated temperatures of about 880 to 999 C. and above, may be used in making such pellets. For instance, high fatty acids, such as stearic acid, and salts of stearic acid, such as zinc stearate or the like, are suitable for use as a lubricant. Carbohydrates, such as dextrose dissolved in water, or camphor dissolved in alcohol, may be used as a binder in making such pellets.

The reducing and sintering treatment carried out at an elevated temperature in the manner described above reduces the oxides and causes the different metal constituents of the different powder particles, 1. e. of nickel and iron powder to mutually diffuse with each other, and the individual powder particles become actually alloyed.

In the particular example referred to herein, the reducing and sintering treatment was performed at a temperature of about 1050 C. for one hour, resulting in sinter cake pellets having a density, of about 2.3 g./cc., with a weight loss of about .5%.

The reduced sintered powder pellets or sinter cake pieces produced in the manner described above are then subjected to a silicon-surfacealloying treatment similar to that applied to the sinter cake pieces made from sponge iron powder as described above. The silicon-surfacealloyed sinter cake pellets obtained by such treatment have about the same density as they had before being subjected to the siliconizing treatment, and they may be comminuted with minimized work hardness into soft silicon-surface-alloyed powder particles having the desired high degree of plastic deformability. By way of example, silicon-suriace-alloyed sinter cake pellets produced in the manner described above, yield upon comminution, a silicon-surface-alloyed iron powder of high softness, and plastic deformability, so that a test bar of such powder compacted under a pressure of 25 t. s. i. without any lubricant or binder, exhibits a modulus of rupture of about 900 p. s. i.

By prolonging the silicon-alloying treatment, or by subjecting the silicon-alloyed sinter cake pieces, such as the sinter cake fragments or sinter cake pellets, to a suitably long additional heat or sintering treatment, the silicon content of the outer layers of the powder particles of such bodies may be caused to diffuse into the interior of the individual powder particles, and thus cause such powder particles to be substantially uniformly alloyed with silicon. Furthermore, such prolonged diffusion treatment will cause the different metals of the individual powder particles, as well as their silicon content, to mutually difiuse, thereby giving the resulting body the characteristic desired alloy composition.

By proceeding in a similar manner with a mixture of oxides of one or more of the metals iron, nickel and cobalt, with one or more of the metals molybdenum, tungsten, chromium, vanadium, manganese, or the oxides of these metals, there may be produced silicon-surface-alloyed soft, plastically, readily deformable powder mixtures of the desired different metals, to wit, iron, nickel, cobalt, molybdenum, chromium, vanadium, and manganese, exhibiting the combined characteristics of the different combined constituents.

It should be noted that when producing silicon-surface-alloyed soft metal powders of the invention from oxides of the desired metals, such as oxides of iron, nickel, cobalt, molybdenum and tungsten, and mixtures thereof, the oxide powders may be pelleted into pellets for the preliminary reducing and sintering treatment. When such pellets are subjected to the combined reducing and sintering treatment of the type described above, the admixed lubricant and binder is decomposed and driven off. As a result, the reduced sinter cake pellets have the required low density and small strength, comparable to the strength of the sinter cake fragments produced by sintering sponge iron powder deposited in a layer of about 4" within the treatment boats in the preliminary treatment of sponge iron powder described hereinbefore. As long as the sinter cake pellets resulting from the preliminary reducing and sintering treatment have a density llnot exceeding about g.,/cc.,. and amodulus. of rupture not exceeding about.6500 p. s. i. the sinter cake pellets will yield SfliCOIl-rBJIOYBd. powders. having the desired high degree, of. softness and plastic deformability as to make. it possible to compact such powders into green compact bodies having a modulusof rupture. of at least.300p..s. i. and higher with a pressure of. only 50g000p. s.. i.

As used in the specification. and. claims, the. expression sinter cake body includes. both sin.- ter cake lumps and sinter cake. pellets of SllfiiCiT- ently low density and strength. that upon comminution of such silicon-alloyed sinter cake bodyinto minute silicon-alloyed powder particles having the desired high degree. of softness and plastic deformability which makes it possibleto. compact such: powders into. green compactedlbodies having a modulus of rupturezofiatleast.3001p s, i. and higher with a pressure of only 50,000. p. s. i.

The principles of the invention described above in connection. with specific exemplifications thereof, will suggest, various. other. modifications and applications of the same... It. is accordingly; desired that the present. invention. shall not be limited: to the specific exemplifications described herein.

We claim:

1. A sintered cake body of metal powder particles which is readily comminutable into soft metal powder particles, the particles of said cake body being composed essentially of at least one metal of the group consisting of. iron containing at most about .2 carbon, of. nickeL. and of cobalt, and mixtures and alloys .ofv said metals, said cake body having been subjectedto a siliconalloying treatment causing. at least the. surface layer of saidparticles to become alloyed with silicon deposited on the particles. from a. siliconv compound at elevated temperatures so that. said bod contains at least 3% of silicon, said cake. body having at most. a density of. about 5. grams per cubic centimeter, whereby said body-may be pulverized into minute silicon-alloyed. powder particleswhich exhibit minimized work hardness andsufficientl-y great. softness so that. when the silicon-alloyed particles. are compacted intoa green body under pressure of about 50.,000. pounds per square inch, said green body exhibits a modulus of rupture of at least 200. pounds per square inch.

2. A sintered cake body of metal powder particles which is readily comminutable into soft metal powder particles, the particles ofv said cake body being composed. essentially of at least. one metal of the group consisting of iron containing at most about 2% carbon, of nickel, and of cobalt, and mixtures and alloys of. said. metals, said cake body having been subjected to a silicon.-

alloying treatment causing at least the surface layer of said particles to become alloyed with silicon deposited on the particles from a silicon compound at elevated temperatures, said cake body containing at least 3% silicon. and having at most a density of about 5 grams per cubic centimeter.

3. A sintered cake body of metal powder particles which is readily comminutable into soft metal powder particles, the particles of said cake body being composed essentially of at least, one metal of the groupconsisting of iron containing at most about .2% carbon, of nickel, and of cobalt, together with at least one metal consisting of molybdenum and tungsten, and of mixtures and alloys of said metals; said cake body having been subjected to a silicon-alloying treatment. causing at least the surface. lay r of. aid particles to become, alloyed with, silicondeposited on the. particles from a gaseous silicon compound at elevated temperatures, said cake body having atmost a density of about 5 grams per cubic centimeter and a modulus of'rupture of atmost 6500 pounds per square inch, whereby said body may be pulverized into minute silicon-alloyed powder particles which exhibit minimized work hardness and sufficiently great softness so that when the silicon-alloyed,particles are compacted into a greenbody under pressure of about 50,090 P n p r. squar inch. aid reen y xhibi amodulus, of. rupture of atleast 200 pounds per square inch.

4. A, sintered cake. body of metalpowder particles which. is readily mminut ble. in S ft. metal, powder particles, the particles of said cake body being ompo ed essentially of. at l st. on metal of. the gro p. onsisting f. iron cont i in at. most about ..2%,. carbon, of, nickel, and of cobalt, together withat. le t one me al of; the group. consisting. of. molybdenum and. t gs en. andofi mixtures. nd, alloys of. ai m tals; sai cake body having been subjecte o a il conalloying treatment causing at. least the surface layer of said particles to become alloyed with silicon deposited on the particles from a silicon compound at elevated temperatures, said cake body containing at least 3%. silicon and having at most a density of about 5 grams per cubic centimeter.

5., The method of providing a readily com pactible and moldable metal powder comprising first. producing readily frangible, porous sinter cake piecescomposed essentially of sinteredmetal powder particles of at, least one metal of the group consisting of, iron containing at most about .2.%. carbon, of nickel, and of cobalt, and mix tures and. alloys. of at least two of said metals; so. that said cake pieces have at most a density of about 4;, grams per cubic centimeter, and thereafter subjecting said cake piecesto a siliconalloying treatment in which silicon from a silicon compound is deposited on the powder particles of said cake, pieces and caused to diffuse into said powder particles so that said powder particles contain at least about 3% silicon, and thereafter comminuting said sake pieces into minute plastically readily deformable metal powder particles, having the deposited silicon alloyed therewith so that when said silicon-alloyed particles are compacted in the absence of a binder into a. green body under a pressure of about 50,000 pounds per square inch said green body exhibits a modulus of rupture of at least about 200 pounds per square inch.

6. The method of providing a readily compactible and moldable metal powder comprising first producing readily frangible porous sinter cake pieces of sintered metal powder particles composed essentially of at least one metal of the group consisting of iron, containing at most about .2% carbon, of nickel, and of cobalt, to gether with up to at least one metal of the group consisting of molybdenum and tungsten, and. of mixtures and alloys of said metals; so that said sinter cake pieces have at most a density of about 4 grams per cubic centimeter, and thereafter subjecting said cake pieces to a, silicon-alloying treatment in which silicon from a silicon compound is deposited on the powder particles of said cake pieces and caused .to diffuse into said powder particles so that the said powder particles contain at least about 3% silicon, and thereafter comminuting said cake pieces into minute plastically readily deformable metal powder particles having the deposited silicon alloyed therewith so that when said silicon-alloyed particles are compacted in the absence of a binder into a green body under a pressure of about 50,000 pounds per square inch, said green body exhibits a modulus of rupture of at least about 200 pounds per square inch.

7. The method of providing a readily compactible and moldable metal powder comprising first producing readily frangible porous sinter cake pieces composed essentially of sintered metal powder particles of the group consisting of iron having at most about .2% carbon, of nickel, and of cobalt, and mixtures and alloys of at least two of said metals so that said cake pieces have at most a density of about 4 grams per cubic centimeter, and thereafter subjecting said cake pieces to a silicon-allo ing treatment in which silicon from a gaseous silicon compound is deposited on the powder particles of said cake pieces and caused to difiuse into said powder particles so that said powder particles contain at least about 3 silicon, and thereafter comminuting said cake pieces into minute plastically readily deformable metal powder particles having the depositedsilicon alloyed therewith so that when said silicon-alloyed particles are compacted in the absence of a binder into a green body under a pressure of about 50,000 pounds per square inch, said green body exhibits a modulus of rupture of at least about 200 pounds per square inch.

8. The method of providing a readily compactible and moldable metal powder comprising first producing readily frangible porous sinter cake pieces composed essentially of sintered metal powder particles of at least one metal of the group consisting of iron containing at most about .2% carbon, of nickel, and. of cobalt, together with up to about 30% of at least one metal of the group consisting of molybdenum and tungsten, and of mixtures and alloys of said metals so that said sinter cake pieces have at most a density of about 4 grams per cubic centimeter, and thereafter subjecting said cake pieces to a silicon-alloying treatment in which silicon from a gaseous silicon compound is deposited on the powder particles of said cake pieces and caused to diffuse into said powder particles so that the said powder particles contain at least about 3% of silicon, and thereafter comminuting said cake pieces into minute plastically deformable metal powder particles having the deposited silicon alloyed therewith so that when said silicon-alloyed particles are compacted in the absence of a binder into a green body under a pressure of about 50,000 pounds per square inch said green body exhibits a modulus of rupture of at least about 200 pounds per square inch.

9. The method of providing a readily compactible and moldable metal powder comprising first preparing a powder mass of powder particles composed essentially of at least one metal of the group consisting of iron containing at least one metal of the group consisting of iron containing at most about .2% carbon, of nickel, and of cobalt, and mixtures and alloys of at least two of said metals, at least some of said powder particles containing the metal in the form of an oxide, heating the powder mass in a reducing atmosphere at a temperature of at least about 700 C. to produce porous sinter cakes having at most a density of about five grams per cubic centimeter, and thereafter subjecting said cake pieces to a silicon-alloying treatment in which silicon from a gaseous silicon compound is deposited on the powder particles of said cake pieces and caused to diffuse into said powder particles so that the said powder particles contain at least about 3% of silicon, and thereafter comminuting said cake pieces into minute plastically readily deformable powder particles of said metals having the deposited silicon alloyed therewith so that when said silicon-alloyed metal particles are compacted in the absence of a binder into a green body having a pressure of about 50,000 pounds per square inch said green body exhibits a modulus of rupture of at least about 200 pounds per square inch.

10. The method of providing a readily compactible and moldable metal powder comprising first preparing a powder mass of powder particles composed essentially of at least one metal of the group consisting of iron containing at most about .2% carbon, of nickel, and of cobalt, together with up to about 30% of at least one metal of the group consisting of molybdenum and of tungsten, and of mixtures and of alloys of said metals; at least some of said powder particles containing the metal in the form of an oxide, heating the powder mass in a reducing atmosphere at a temperature of at least about 700 C. to produce porous sinter cakes having at most a density of about five grams per cubic centimeter, and thereafter subjecting said cake pieces to a siliconalloying treatment in which silicon from a gaseous silicon compound is deposited on the powder particles of said cake pieces and caused to diffuse into said powder particles so that the said powder particles contain at least about 3% of chromium, and thereafter comminuting said cake pieces into minute plastically readily deformable powder particles of said metals having the deposited silicon alloyed therewith so that when said silicon-alloyed metal particles are compacted in the absence of a binder into a green body under a pressure of about 50,000 pounds per square inch said green body exhibits a modulus of rupture of at least about 200 pounds per square inch.

GEORGE STERN. S. J. SINDEBAND. J. P. SCANLAN.

References Cited in the file of this patent FOREIGN PATENTS Country Date Great Britain Feb. 12, 1948 OTHER REFERENCES Numb er 

1. A SINTERED CAKE BODY OF METAL POWDER PARTICLES WHICH IS READILY COMMINUTABLE INTO SOFT METAL POWDER PARTICLES, THE PARTICLES OF SAID CAKE BODY BEING COMPOSED ESSENTIALLY OF AT LEAST ONE METAL OF THE GROUP CONSISTING OF IRON CONTAINING AT MOST ABOUT .2% CARBON, OF NICKEL, AND OF COBALT, AND MIXTURES AND ALLOYS OF SAID METALS, SAID CAKE BODY HAVING BEEN SUBJECTED TO A SILICONALLOYING TREATMENT CAUSING AT LEAST THE SURFACE LAYER OF SAID PARTICLES TO BECOME ALLOYED WITH SILICON DEPOSITED ON THE PARTICLES FROM A SILICON COMPOUND AT ELEVATED TEMPERATURES SO THAT SAID BODY CONTAINS AT LEAST 3% OF SILICON, SAID CAKE BODY HAVING AT MOST A DENSITY OF ABOUT 5 GRAMS PER CUBIC CENTIMETER, WHEREBY SAID BODY MAY BE PULVERIZED INTO MINUTE SILICON-ALLOYED POWDER PARTICLES WHICH EXHIBIT MINIMIZED WORK HARDNESS AND SUFFICIENTLY GREAT SOFTNESS SO THAT WHEN THE SILICON-ALLOYED PARTICLES ARE COMPACTED INTO A GREEN BODY UNDER PRESSURE OF ABOUT 50,000 POUNDS PER SQUARE INCH, SAID GREEN BODY EXHIBITS A MODULUS OF RUPTURE OF AT LEAST 200 POUNDS PER SQUARE INCH. 